The lithium ion polymer batteries fabricated with a solid polymer electrolyte having a high ionic conductivity have advantages to solve the problems of the exudation of electrolyte solution and explosion, which have occurred in the lithium ion cell using liquid electrolyte. Also, if a solid polymer electrolyte is employed, it is possible to design various shapes of cell as well as to simplify the process of manufacturing the lithium ion polymer batteries. Since the lithium ion polymer battery has little self-discharge up to 5% and no memory effect, it is spotlighted as a next generation battery. A method of preparing a lithium ion polymer battery has been developed in many ways.
U.S. Pat. No. 5,491,041 discloses a solid state rechargeable electrochemical cell comprising a Li intercalating composite cathode, a solid polymer electrolyte separator, and a composite anode containing graphite, a polymer electrolyte and 12-crown-4-ether. The electrodes of the U.S. patent above are prepared by coating a current collector layer such as aluminium or copper foil with a cathode or an anode active material. However, the cell has a poor cyclability due to being separated the active material from the current collector layer during charge/discharge cycles. Further, the capacity of the cell per unit area cannot be increased due to the limitation to the thickness of electrodes.
U.S. Pat. No. 5,540,741 discloses a method of making a battery structure comprising forming a solid state positive electrode of a polymer, a plasticizer and an active material, forming a solid state negative electrode of the same polymer, a plasticizer and an active material, forming a separator membrane of the same polymer and a plasticizer, bonding said electrodes and said separator membrane to form a unitary cell structure, extracting a portion of said plasticizer from at least said separator membrane, and activating said battery by reswelling at least said separator membrane by contacting said unitary battery structure with an electrolytic salt solution. This method of the patent has an advantage that most of the steps for preparing the battery structure can be carried out under the atmosphere so as to avoid using an electrolyte solution which is sensitive to the moisture, and that the activating step is carried out under the anhydrous atmosphere. However, the battery prepared by the method has an exudation problem of liquid electrolyte, because the polymer electrolyte does not retain the liquid electrolyte for a long period. Also, it is not easy to retain a sufficient amount of the liquid electrolyte in the microporous polymer membrane.
Therefore, the present inventors have developed a lithium ion polymer battery to overcome the shortcomings of the conventional batteries and a method of preparing the same.